1. Field of the Invention
The present invention relates to hexafluoroacetone and a method of preparation for same.
2. Description of Related Art
Many organic compounds which are polyfluorinated or perfluorinated are quite valuable. Some of the many uses of one such simple perfluorinated ketone, hexafluoroacetone [HFA], were reviewed a number of years ago. The varied uses include polymeric monomers or intermediates for monomers as well as solvents, chemicals and drug products such as sevoflurane. However, the commercial availability of HFA is limited to only trivial amounts.
There are several reactions available which allow one to prepare HFA. Bigelow first described the direct reaction of acetone with elemental fluorine. The efforts of others to control the extreme reactivity of this reaction were partially successful, but the cost of elemental fluorine is too high to be economically viable.
The halogen exchange reaction of hexachloroacetone with hydrogen fluoride [HF] has been described in French Pat. 1,372,549, with other variations following. The reaction is performed in the vapor phase over a suitable catalyst. The preferred catalyst consists of a trivalent chromium compound. This reaction suffers from the high boiling point of hexachloroacetone which makes it hard to vaporize at industrially preferred pressures of 100 to 250 psig. Also conversion is not complete and toxic chlorofluoroacetone by-products are produced. Great care must be taken to remove these completely from the product. Finally, the product is isolated as a fluorohydrin, a compound formed from the ketone and hydrogen fluoride rather than in the ketone itself. The HFA fluorohydrin is an adduct which is sufficiently stable so that it can be distilled without decomposition. Hydrogen fluoride can be removed by supercritical distillation process. Alternatively, hydrogen fluoride can be removed by scrubbing with sodium fluoride, sulfur trioxide or NaBO.sub.2. All of these steps make the fluorohydrin undesirable as an intermediate in the ketone manufacture.
Suitable fluoro-olefins can be oxidized to yield HFA. U.S. Pat. No. 2,617,836 teaches the use of perfluoroisobutylene, a by-product from the production of hexafluoropropylene. However, perfluoroisobutylene's extreme toxicity precludes its shipment and handling. The oxidation of hexafluoropropylene is taught by Carlson in U.S. Pat. No. 3,536,733 to yield interalia HFA. The similar boiling points of the product, unreacted starting material and by-products which include hexafluoropropylene oxide and pentafluoropropionyl fluoride make separation tedious.
Once purified, hexafluoropropylene oxide can be isomerized to HFA, as in U.S. Pat. No. 3,213,134 with a catalyst of antimony pentafluoride. The added separations and isomerization make this preparation less convenient.
Oxidation of hexafluorothioacetone dimer is illustrated by Middleton as a convenient preparation of HFA, but the dithiane must first be prepared from hexafluoropropylene and thus adds another step to the process and additional waste in the form of sulfite/sulfate.
Direct oxidation of highly fluorinated hydrocarbons with oxygen and chlorine as initiator has been described by Haszeldine. The products are straight chain acyl halides or acids of the corresponding fluorinated hydrocarbon. Ketones were not obtained as products. A suitable hydrofluoropropane for direct oxidation to HFA would be 1,1,1,3,3,3-hexafluoropropane, R236fa. High temperature reactions of R236fa in the range of 550-585.degree. C. have been previously described by McBee. However, the paucity of reported detail masks the fact that poor conversions are a result of poor reactivity. Thus, a highly reactive initiator such as elemental fluorine is required to oxidize R236fa directly to HFA, as illustrated in U.S. Pat. No. 5,629,460.
A disadvantage of the above fluorine initiated direct oxidation is that water is produced as a by-product. Water combines with HFA to form a hydrate, sesquihydrate and trihydrate, depending on the amount of water present. These hydrates are stable and may be sublimed or distilled without liberation of anhydrous HFA. Water must be removed from these hydrates by use of a water sequestering agent such as P.sub.2 O.sub.5 or SO.sub.3.
Thus, what is needed is a means to prepare HFA on an industrial scale from materials that are readily available, of low toxicity and easily handled. The reaction should give high conversion and yield in one step and be largely free of by-products that produce stable adducts or make separation difficult.